In the construction of a certain type of instrument it is necessary to have a very long optical cable, on the order even of from a few hundred meters to 2500-3000 meters, which is able to withstand high temperatures--even of more than 500.degree. C.--and high pressures of up to 250 bar, and which is water-and gastight and also resistant to corrosion.
Individual optical fibers of quartz of low attenuation and indefinite length, without covering, are available, which are suitable from an optical standpoint but are not suitable in themselves for certain applications.
The first object of the present invention is a protected optical fiber cable which is formed of at least one optical fiber contained in a sheathing consisting of a metallic capillary tube. The transverse dimension-particularly the diametral dimension--of the capillary metallic tube is, in practice, of a larger order of magnitude than the optical fiber or of each optical fiber contained in it. The optical fiber may have diametral dimensions on the order of 100 .mu.m and the metallic capillary tube a diameter on the order of 1.7 mm.
A second object of the invention is a process of producing an optical fiber cable as defined above having at least one optical fiber within a capillary metallic tube.
In accordance with this process, a flow of a fluid is established in said tube in order to entrain the optical fiber into the tube.
In practice, an initial length of optical fiber is inserted into the capillary metallic tube and thereupon a flow of a liquid--such as water, distilled water, or the like--is established under pressure in the tube in order to effect a progressive pulling of the optical fiber into the tube.
Still another object of the invention is an apparatus for producing an optical fiber cable such as defined above by the use of the aforementioned process. This apparatus comprises a hollow body containing a reel on which the optical fiber is wound, a mouth which widens towards the inside of the hollow body in order to receive the fiber as it is unwound, a chuck, to engage in watertight manner the end of the metallic capillary tube, and a pump adapted to introduce into the inside of said hollow body a liquid under high pressure which flows through the mouth and the tube.
Advantageously a brake is provided on the reel in order to adjust the unwinding of the optical fiber entrained by the flow of fluid; said brake may be of the magnetic induction type, with a magnet facing a disk or ring of copper or the like.
The apparatus may comprise a means for measuring the unwinding of the optical fiber, in particular a means for measuring the revolutions of the reel, in the form of a magnetic ring driven along by induction by the reel, without contact.
The apparatus may furthermore comprise within the hollow body at least one window of transparent thickness in order to monitor the inside of the cavity.